In integrated circuits, there is often a need for discretionary connections which, once established, are permanent. Such discretionary connections are useful, for example, to program read-only memories, for swapping redundant circuit elements for defective circuit elements in memory arrays, for selecting useful circuits in wafer scale integration, and for enabling features or disabling features in integrated circuits. Most often, discretionary connections are made with either fusible links or antifusible links.
An antifusible link, when fabricated, provides an open connection. Antifuse links consist of two conductor and/or semiconductor materials having some kind of a dielectric or insulating material between them. A silicon nitride dielectric layer interposed between two conductively-doped silicon conductors is one common type of antifuse. During programming, selected antifuse links are shorted to provide a closed connection by applying a voltage across the dielectric layer that exceeds its breakdown voltage.
A fusible link, on the other hand, provides a closed connection when fabricated. The fusible link may be selectively melted to provide an open connection. In some cases, the fusible link is melted by passing an electric current through it which is of sufficient magnitude to cause the element to melt. In other cases, a laser beam is used to melt the link.
The use of laser-blown fuses poses several problems. Not infrequently, the blowing of a fusible link does not result in an adequate disconnection between two fuse-linked nodes. This problem may be the result of several factors. First, dielectric layers, which are often deposited on top of the fusible elements are not uniformly thick from wafer to wafer. Even on the same wafer, dielectric layer thickness may vary from the edge to the center. Therefore, the energy required to cleanly sever a fusible element may vary from device (also referred to as a die or chip) to device. If a fusible element is not cleanly blown such that a path of acceptably high resistance exists between the formerly interconnected nodes, it is generally advisable to discard the device, as it is usually undesirable to blow a fuse more than once in the same location. In any case, a relatively large gap must be blown in the link in order to prevent it from later becoming closed during operation of the device, through transmigration of conducting material near the gap into the gap region. This phenomenon of a blown fuse element "healing" itself is more likely to occur under conditions of high voltage and elevated temperature. The phenomenon is particularly problematic, because parts that are deemed to be good during testing immediately following manufacture may degrade in the field to the point where the selected option or replaced redundant circuit element is no longer selected consistently. Once the part becomes unreliable, it must be replaced. It is a generally accepted maxim that the more current that passes through a fusible link, the greater the likelihood of a future transmigration-induced failure.
Fuse circuits, such as the one depicted in FIG. 1 have been commonly used to make discretionary, permanent connections in integrated circuits. For example, such a circuit may be used to effect repairs in VLSI and ULSI random access memory (DRAM) arrays. In such a case, repairs are performed by disabling defective array elements (i.e., one or more rows and/or columns) and enabling functional redundant array elements in such a way that the enabled array element will be addressed in lieu of the disabled array elements. The fuse circuit is replicated many times within the array, with each circuit being associated with a group of rows or a group of columns, rather than with a single row or a single column. Such an arrangement represents a compromise between the competing goals of minimizing circuit space dedicated to repair circuitry and the ability to repair the maximum number of integrated circuit devices.
Still referring to FIG. 1, when the group of array elements associated with an individual repair circuit are fully functional, node A is maintained at ground potential (typically referred to as V.sub.ss) through the unblown fuse element FE. The output of the repair circuit in such a case is high. However, if one or more of the array elements associated with the repair circuit are determined to be defective during testing, fuse element FE is blown with a laser pulse in order to isolate node A from ground. Device Q1 then pulls node A to near power supply voltage (typically referred to as V.sub.cc), with the result that the output of the circuit is, now, low. The output feedback path P1 then latches transistor Q2 to an "on" state, thus ensuring that node A is held near V.sub.cc. The output signal, when low, enables address rerouting for the defective group of circuit elements.
Still referring to FIG. 1, if fuse FE is not completely blown, or if the open circuit partially "heals" itself, a leakage path through fuse element FE will exist. Node A will no longer be maintained near V.sub.cc, but rather at some voltage between ground and V.sub.cc. As a consequence, the circuit will be either unstable or completely nonfunctional. The problem will be particularly acute for low V.sub.cc levels, since transistor Q1 will not have sufficient drive to compensate for the leakage through fuse FE.